A conventional optical film as shown in FIG. 1 comprises a supporting layer 5 and a prismatic layer 6 formed on the supporting layer 5.
When an incoming light ray1 enters the prism 61 of this first prior art, ray1 will be reflected from an inside surface of a first dihedral face 6a and projected towards a second dihedral face 6b of the prism 61 through a light path having an incidence angle θ4 from a normal line N1 perpendicular to point A on the second face 6b. Then, an outgoing light ray1′ will be refracted outwardly from the second face 6b at point A through a refracted angle θ5 from the normal line N1. The outgoing light ray1′ has been refracted outwardly above an apex of a neighboring prim 61a and can not be picked up by the neighboring prism 61a, thereby causing light loss of the optical film.
U.S. Pat. No. 5,771,328 disclosed a light directing film having variable height structured surface in order to inhibit visible optical coupling when adjacent sheets of light directing film are nearly contacted. An example of this second prior art (U.S. Pat. No. 5,771,328) is illustrated in FIG. 2 of this application. In this second prior art, the taller prisms Ph of the first sheet S1 of film will limit the physical proximity of a second sheet of film S2 when contacted with the structured surface of the first sheet of film S1 to reduce the likelihood of undesired optical coupling (on the shorter prisms Ps) between the structured surface of the first sheet S1 with the adjacent second sheet of film S2.
However, the variations in the height of the shorter prisms Ps as shown in FIG. 2 may still appear as visible lines on the surface of a film, which are visible by human eyes. Therefore, the optical defects as found in the second prior art are still expected to be overcome.
The present inventor has found the drawbacks of the prior art, and invented the present optical film to overcome the drawbacks of the prior arts.